The story behind solar startup Alta Devices’ innovation

Alta Devices has garnered high-profile investors (Kleiner, NEA, GE, Dow Chemical), but it’s been fairly quiet about the technology it has developed to bring in those investors, as well as its plans for commercializing that tech. But after chatting with the Silicon Valley startup’s CEO, Christopher Norris, on Monday, we have a lot better idea on both of these fronts.

Alta Devices is combining the material gallium arsenide, which is already pretty efficient at converting sunlight into electricity, with a process that can harvest additional energy from the material. The ultra-efficient gallium arsenide cell is also super-thin yet able to convert more sunlight into energy than silicon and cadmium telluride cells: the two most widely used solar cell materials today.

An Alta cell is about 1 micron thick compared with 150-200 microns for silicon cells and roughly 15 microns for cadmium-telluride, Norris said. Making thinner cells is a goal for all cell developers because it cuts costs.

Alta has achieved 28.2 percent efficiency with its cells, an efficiency that breaks the previous record of 26.4 percent for gallium arsenide cells and has been verified by the National Renewable Energy Laboratory, the company said. At 28.2 percent, the cells are far more efficient than the most efficient silicon solar cells available on the market today. SunPower is making solar cells with 22.4 percent efficiency and panels at roughly 20 percent efficiency, the company announced earlier this month. First Solar, the world’s largest cadmium telluride cell maker, is making panels at 11.7 percent efficiency. Panel efficiencies tend to be 10-12 percent less than cell efficiencies.

The startup chose to use gallium arsenide because it is naturally more efficient at converting sunlight into electricity compared to silicon and other materials. But the chief reason gallium arsenide isn’t commonly used in solar cells today is its cost. Because of its high price, solar cell developers typically have used very little of it and only in combination with other materials to develop a type of multilayer cells that are found in solar electric systems that use mirrors to concentrate sunlight by hundreds of times onto the cells.

Alta’s technology was developed out of research from co-founder Eli Yablonovitch, a professor at UC Berkeley, who worked on a technique called “epitaxial lift-off,” which makes it possible to create very thin gallium-arsenide cells with internal structures that can squeeze more power from the sunlight that enters the cells. The tech enables a photon to produce more electricity after it has entered a solar cell, because when photons enter a cell, they tend to knock loose the electrons in the material and create holes that can hold those electrons. The free electrons can be collected and create current, which then combines with voltage from the electrical field in the cell to form power.

Not all the electrons in the process can be collected to create electricity, but the electrons that can’t be collected can be formed into photons, and can be used to knock loose more electrons to increase the overall power output of the cell, Norris said. The process of an electron becoming a photon again (after rejoining the hole) is called “radiative recombination. “It’s called photon recycling. What we focus on is making use of the primary (energy) loss mechanism inside the material,” Norris said.

Alta is still in the product development stages: It concocted the best cells in its lab. The company plans to build a pilot factory in Sunnyvale, Calif., and start producing cells next year, Norris said. The cells will be entombed in plastic materials to keep moisture out and make the cells flexible for many applications, and Alta wants to sell cells to companies who will assemble them into panels or embed them in roofing and other building materials. Alta also will likely produce its own panels as well.

The plan is to start commercial shipment in 2013. “Nobody has commercialized a gallium arsenide thin film yet. We are going to be the first to do that.” Earlier this year, Alta said it had raised $72 million from investors including Kleiner Perkins, Dow Chemical and a joint venture among GE, ConocoPhilips and NRG Energy.

In Monday’s announcement, Alta Devices mentioned research by one of its co-founders and Caltech professor Harry Atwater, who is presenting a paper about silicon microwires. The silicon microwire research is actually not related to what Alta Devices is working on, Norris said.